Low level rubella immunity detected by ELISA and specific lymphocyte transformation

The level of humoral and cell mediated immunity in persons with low or undetectable hemagglutination-inhibition (HAI) rubella titers was investigated by ELISA, IgG presence in sucrose gradient separated serum fractions and lymphocyte transformation. The study population consisted of persons with stated history of natural rubella infection and rubella vaccinees. Persons with natural rubella infection and HAI titers of 1:8 or +/- 1:8 (i.e., incomplete HAI at serum dilution of 1:8) were all ELISA positive and the stimulation index (SI) of specific lymphocyte transformation was higher than 2.5. Among the 20 persons with HAI titers of < 1:8, 8 were found to be ELISA positive and their SI was also > 2 and IgG was detected in their serum. Rubella vaccinees with HAI titers of 1:8 or +/- 1:8 were likewise ELISA positive. Their SI was lower: none higher than 3, but none lower than 1.5. Among 23 HAI negative vaccinees, 14 were found ELISA positive. This serum fraction contained IgG and the SI was > 1.5. It appears that ELISA test is able to detect antibodies where the HAI test fails. The positive outcome of ELISA test in this case was confirmed by the presence of IgG in serum fractions and by the lymphocyte response to rubella specific stimulation.